This invention relates to a method of forming a pattern of conductor runs on a sheet of dielectric material.
A so-called hybrid integrated circuit comprises a sheet-form substrate of ceramic material having one or more electronic components, such as monolithic integrated circuit chips, adhered to an upper surface of the substrate. Each electronic component has at least two terminals. The substrate may be composed of multiple sheets of ceramic material, secured together by sintering. Conductor runs are formed on the upper surface of each sheet. The conductor runs on each sheet interconnect contact pads on that sheet. A contact pad on the upper sheet may be wire bonded to a terminal of an electronic component, connected to a contact pad of one of the lower sheets through a via in the upper sheet (and possibly one or more vias in the lower sheet(s)), or connected to a conductor run of a printed circuit board. A contact pad on a lower sheet may be connected to a contact pad of another sheet through one or more vias.
It is known to use a screen printing process for providing a desired pattern of conductor runs on a ceramic sheet. In this process, a conductive ink is printed onto the ceramic sheet when it is in the unfired, or green, state. The green ceramic sheet comprises alumina powder in a matrix of an organic binder material, such as the material sold by Monsanto Company under the trademark BUTVAR 60, and is flexible and quite tough. The ink comprises a suspension of tungsten powder in an organic vehicle, and the pattern is defined by conventional screen printing techniques. After printing, the green ceramic sheet is heated to a temperature of about 800.degree. C. in air in order to burn off the organic binder material and the organic vehicle, which decompose into carbon dioxide and water, and is then fired at a temperature of about 1700.degree. C. in an atmosphere of hydrogen. During the firing operation, the tungsten is sintered to the ceramic material.
This conventional process is not suitable for manufacture of a small number of sheets, such as might be required for prototyping a hybrid circuit. In particular, it is not economical to manufacture a printing screen if only a few sheets are to be printed.
It is known to form a visually-distinct pattern on a receiver sheet made of paper by placing carbon paper, which comprises a carrier foil having a pigment-laden wax adhered to one surface thereof, with its coated surface in contact with the receiver sheet and applying pressure to the opposite surface of the foil in accordance with the desired pattern. The pigment-laden wax is detached from the carrier foil at locations at which pressure is applied and becomes attached to the receiver sheet.
A thermal wax printer is known in which a receiver sheet is passed between a back-up drum and a print head comprising a linear array of resistors. A transfer tape, comprising a carrier foil having a coating of pigment laden wax on one surface thereof, is positioned between the receiver sheet and the print head, with the wax towards the receiver sheet. As the sheet is advanced between the back-up drum and the transfer tape, the resistors are selectively activated. When a resistor receives current, it becomes hot and the wax on the area of tape that is immediately beneath the resistor is transferred to the receiver sheet. By appropriate selection of the resistors that are stimulated in relation to the advancement of the receiver sheet, a desired pattern of pigment-laden wax is formed on the receiver sheet.